Title

Author

Date of Award

Degree Type

Degree Name

School

School of Natural Sciences

Faculty

Faculty of Computing, Health and Science

First Advisor

Dr Christine Hanson

Second Advisor

Dr Glenn Hyndes

Third Advisor

Dr Mat Vanderklift

Abstract

The application of stable isotope ratios to food web studies is increasing, and the use of generalised discrimination values (0.4±1.4%0 for δ13C and 3.4±1.1%0 for δ15N), which are being widely applied to many studies, may not be valid. The broad objective of this study was to evaluate the assumption that these discrimination values are applicable to a range of benthic marine consumers, and therefore appropriate to be used in trophic analyses using carbon (C) and nitrogen (N) stable isotopes in marine food webs. The first aim was to determine if there were differences in discrimination values: (1) among different groups of macroalgae when fed to specific consumers; and (2) among different types of consumers when fed on the same type of algae. The feeding treatments used representatives of brown, fleshy red, calcareous red and green macroalgal food sources, which were fed to three types of consumers: the sea urchin Heliocidaris e1ythrogramma, the hermit crabs Paguristes purpureantennatus and Calcinus dapsiles, and the gastropod Turbo torquatus. Food sources and consumers were collected from the field, and each food source was subsequently fed to each consumer in a series of controlled aquaria experiments. Replicate individuals of each consumer were sampled at regular intervals up to 56 days after initiation of the experiments. Muscle tissue was processed and analysed using mass spectrometry to determine δ13C and δ15N values. In accordance with the feeding treatments, starvation treatments were also conducted. The results showed a general trend of no change in δ13C and δ15N isotope ratios for the invertebrate consumers over time, and this was also the case for the starvation treatments. Observations during the experiments indicated that animals were active and consuming the algae in the feeding treatments compared to the starved invertebrates which remained relatively immobile. Constant feeding by invertebrates suggests that discrimination values derived from the experiment are likely to be realistic. Results indicated that δ13C and δ15N discrimination differed among groups of algae, particularly for carbon. Average δ13C discrimination values were 1.56%0 for brown algae, 2.31 %0 for fleshy red algae, 3.38%0 for calcareous red algae, 1.46%0 for green algae, while average δ15N discrimination values were 2.6­0%0, 2.03%0, 2.53%0, and 2.93%0 for the respective algal groups. δ13C and δ15N discrimination also differed among consumers when fed on the same food source. Average δ13C discrimination values were 2.67%0, 2.70%0, 2.58%0 and 1.31 %0 for the urchin Herythrogramma, the hermit crabs P. purpureantennatus and C. dapsiles and the gastropod T torquatus, respectively, while average δ15N discrimination values were 3.14%0, 3.80%0, 2.15%0 and 1.31%0 for the respective species. Thus, the results of this study indicate that different discrimination values are exhibited for different algal food sources and by different consumers under the environmental conditions within this study. Furthermore these values differed from the assumed discrimination values that are applied to many stable isotope studies. In many isotope-based food web studies, mixing models are utilised to quantify the possible contributions of different food sources to a consumer's diet. This process has better outcomes than traditional gut content analysis, as it reflects actual assimilation of food sources rather than what material is found in the gut tract. The second aim was to compare results of the Isosource mixing model when using either assumed or experimentally-derived discrimination values to evaluate the contribution of the different macroalgal species to the natural diets of marine invertebrates. The assumed discrimination values of 0.4%0 for δ13C and 3.4%0 for δ15N were applied to the food source stable isotope dataset as input data for Isosource, and compared to results when the experimentally derived values were applied to the same dataset. Results from Isosource were not possible using the assumed values, due to the isotope ratios of consumers sitting outside the range of sources. However when the experimentally derived discrimination values were applied, the model ran successfully. These mixing model results indicated that brown, red and green algae all contributed to the production of consumers, but there were differences in the relative contributions of these food sources among the consumer species examined. The current study has quantified C and N discrimination for key marine invertebrate and algal types, and particularly for C these values were found to be higher (1.46-3.38%0) than those generally applied, i.e. 0.4 ± 1.4%0. The assumed discrimination values are not applicable across all food sources or consumer groups. The consequences of applying incorrect discrimination values include preventing any sources of production to be established, or leading to erroneous conclusions about an organism's trophic level or food source.